The present invention relates to electro-optical imaging display systems and, more particularly, to infrared systems and systems employing two or more light beams and associated electronics.
Scanning infrared sensors are known in the art. In such, sensors, an electro-optical detector converts incident electromagnetic radiation from a viewed field into an electrical signal which is then processed by a signal processor for display in an imager. A telescope unit, which can be directed to point or sight in a desired direction, receives the radiation and directs it onto a scanner having one or more reflective surfaces thereon. Motion of the scanner causes the beam to be swept across the sensitive detecting surface of a detector.
In certain imaging situations, it is desirable to view the subject through a narrow field of view and a wide field of view, the wide field of view providing a background reference useful in signal processing operations including the extraction of signal from noise. A problem arises in that with presently available imaging equipment, two separate sensors including their respective scanners, cryogenic coolers, optics, and imaging electronics must be employed to provide the two fields of view. This introduces unwanted weight, complexity and cost to the imaging and display system.
These problems were solved in the invention described in U.S. Pat. No. 4,574,197 filed Mar. 24, 1983 entitled "Dual Field of View Sensor" by W. W. Kliever In that invention, a multifaceted polygon wheel scanner shares the inputs from wide and narrow field of view optics and the outputs to detector, one or more displays, and associated electronic and cooling mechanisms. It comprises a scanning infrared sensor or the like employing a scanner, a detector, a beam clipper or equivalent, and a signal processor. A first optical assembly is employed to direct light entering from a first line of sight against the reflective surfaces of a rotating polygon scanner so that the light is sequentially reflected from the facets of the scanner across the detector in a sweeping scan pattern to produce a signal therefrom. A second optical assembly directs light entering from a second line of sight against the reflective surface of the scanner at a point displaced one-half its facet angle for enabling the detector and signal processor and any other mechanisms to be time-shared Thus, light from the first and second lines of sight are interleaved and swept, one at a time, across the detector.
The polygon wheel scanner utilized in the invention in U.S. Pat. No. 4,574,197 accommodates a small telescope exit pupil and, if a large unit pupil were desired, the polygon facetes would have to be appropriately enlarged, thus requiring a larger polygon diameter and wheel. Where space, weight and cost are important considerations, such enlargement has obvious limitations.
In addition, if the scanner must be stopped and started instantaneously, the mass of the scanner must be kept to a minimum. The multifaceted polygon wheel described above has mass sufficiently large to require several seconds for the wheel to stop or start. Thus, its mass militates against its use where instantaneous starting and stopping is required, such as a transition from a scanning mode for a narrow field of view to a wide field employing another mirror.